Switches/buttons in good shape, Diving into debugging of the machine state

TESTED SWITCH CONTACTS ON MACHINE

The pushbuttons and switches were all in pretty good shape, I imagine they were cleaned up by the prior restorers. Everything was good so I could move on to debugging of logic.

DEBUGGING FIRST PROBLEM

I hooked up my oscilloscope to begin chasing down the signals involved in the state machine that controls the processor starting, stopping, stepping and other basic functions. I found a signal behaving quite oddly. This signal, the Delay state flipflop, is on of a chain of three that start the machine running - Advance, Delay and Run. 

Main clock above and abnormal Delay FF signal below

As you can see from the signals, with the top yellow trace showing the main system clock at 2.25MHz, the bottom blue trace is oscillating much faster than any clock in the system. It appears to be about 12.72Mhz. I talked with the last restorer who was dealing with debugging this area of the machine. 

He mentioned that he had swapped in a card from a different 1130 system they used for spare parts. The 1130 did not work with that card either, but the symptoms were different. This suggests that at least one of these cards is bad, if not both. 

I decided to debug the card out of the machine, to simplify things considerably. So many machine functions spread across different boards and backplanes are involved in the central state machine behavior that it might require an unwieldy number of observations before I come across the problem or prove that this card is bad.

SETTING UP MY CARD DEBUGGING RIG

I have a handy appliance with SLT sockets and a breakout of each pin that is easily accessible. The SLT logic requires three power rails - +3, -3 and +6V - which I can easily generate with my bench power supplies including current limiting to avoid any damage during testing.

My SLT card debugging rig with the suspect card inserted

This card receives the oscillator at 2.25MHz and passes on clock pulses based on the run state machine inside. The double size SLT card has 38 input or output signals in addition to the power connections. The inputs are a number of machine conditions, the Prog Start and Reset pushbuttons, the oscillator input and similar state that determines whether to run, wait or single step. 

My function generator will produce the oscillator square wave that will drive the system. While I could slow down the oscillator rate that is not how the 1130 does single stepping style operations so I won't either. 

I can use pullup or pulldown resistors to set the logic levels of all the inputs and then observe the outputs to determine if there are any defective gates on the card. Conveniently I can debug both of the cards using the same setup.